1. Field of the Invention
The present invention relates to a mask for use in transferring a very fine pattern of a semiconductor device with an X-ray or electron beam, a mask inspection apparatus and a mask inspection method.
2. Discussion of the Background
With a recent higher integration density of the semiconductor device, the circuit pattern of its associated LSI elements is more and more microminiaturized. For the microminiaturization of such a pattern it is required that not only its line width be narrowed but also the dimensional accuracy and positional accuracy be enhanced. In order to meet such requirements, the development of the associated exposure techniques have been rapidly progressed. Among these techniques, the exposure technique using the X-ray has been regarded as a promising technique of the next generation behind the current mainstream exposure technique using an ultraviolet radiation.
The currently developing X-ray exposure technique using a synchrotron radiation light provides an exposure system in which the light is conducted to a stepper through a mirror for enlarging an exposed area and a beam path constituted by a Be thin film, etc., serving as a vacuum partition wall and one-to-one proximity exposure is made using a 100%-magnification mask.
For the X-ray exposure light, therefore, a higher accuracy mask than the existing exposure-reducing mask using the ultraviolet radiation is required and a very stricter level of inspection is required. The mask inspection method using the existing ultraviolet light cannot solve the stricter conditions because of the resolution limit of an optical system. Therefore, a growing demand is made to develop a mask inspection method using a charged particle beam, such as an electron beam for obtaining a higher resolution.
FIG. 1 shows a general arrangement of the mask inspection apparatus. In this arrangement, an electron beam 52 is generated from an electron beam generator 51 and irradiated onto an X-ray mask 70. By doing so, secondary electrons are emitted from the surface of the X-ray mask 70. The secondary electrons are detected by a detector 54. A comparator 55 converts a pattern of the x-ray mask 70 to an image on the basis of the detection signal and compares it with mask data 56. By doing so, a pattern defect is picked up. Another method is also proposed for detecting a transmitted electron from the mask 10 instead of such secondary electrons and converting a pattern of the mask 70 to a corresponding image.
However, such an existing inspection method has the following problems. With the pattern microminiaturized, a defect is also microminiaturized. In order to pick up such a microdefect, the diameter of the electron beam has also to be micro-sized. It is, therefore, unavoidable that the inspection time, that is, the scanning time, over a whole mask is increased. It sometimes take 10 hours, for example, to inspect one sheet of a mask. Stated in easier terms, the inspection time becomes longer in proportion to one divided by a square of the beam size.
For the existing mask inspection apparatus primarily inspection the defects of the X-ray mask, it has taken a very long time to inspecting the mask in the case where more exact inspection has to be made on a micropattern.
In the case where the mask is checked with the use of an electron beam, the following problem also arises due to the structure of the X-ray mask. That is, the X-ray mask is so formed on a very thin film of 2 .mu.m with an X-ray absorbing material corresponding to a transfer pattern formed thereon and has a small heat capacity. When, therefore, a charged particle beam for inspection is irradiated in a large quantity on the mask, an energy of the X-ray beam absorbed at a heat-directing area is converted to heat, thus causing a thermal deformation in the mask due to a rise in temperature there. As a result, some trouble occurs on inspection at a pattern coordinate. If an attempt is made to increase a current density so as to improve the throughput, contrast, S/N ratio, etc., in particular, an extremely great thermal deformation occurs in the mask, a state not practically tolerable in this case.
In the case where inspection is made on the X-ray mask with the use of a charged particle beam, such as an electron beam, there occurs a not-negligible event due to a heat generation resulting from an energy absorbed in the mask and an attendant thermal deformation there.